docs: def usable_id(cls, id, datacenter=None):
          """ Retrieve id from input which can be label or id."""
          try:
              qry_id = int(id)
          except Exception:
              # if id is a string, prefer a system disk then a label
              qry_id = cls.from_sysdisk(id) or cls.from_label(id, datacenter)
  
          if not qry_id:
              msg = 'unknown identifier %s' % id
              cls.error(msg)
  
          return qry_id